Officials responsible for the accountability and physical security of materials have recognized for a number of years the need to develop a secure, tamper-resistent indicating seal whose integrity and unique identity could be established in the field without removal or disassembly. A need has been also identified for a secure sealing system that could be continously and remotely monitored.
A new class of sealing devices, a fiber optic seal, has been described in which the typical sealing wire or tape was replaced with a glass fiber optic bundle. The seal was made by enclosing the two ends of the fiber bundle in a frangible, stressed glass clasp or by removing the protective plastic jacket from the bundle ends and holding the twisted glass fiber ends together with a metal collar. The optical integrity and continuity of the fiber optic loop could be checked after assembly by noting the transmission of light through the bundle. The unique identity or fingerprint of the seal could be established at the time the seal was assembled in the field by recording the random pattern of the ends of the glass fibers in the bundle. It was suggested that, if part of the bundle was illuminated, a random pattern of dark and illuminated fiber ends would be produced which could be photographed in the field using a portably hand-held photomicrographic instrument. A direct comparison of a negative taken at the time of the assembly of the seal with a positive print taken at a later date would provide the highest level of confidence if it must be established that a seal left unattended and unexamined for a significant interval of time has not been compromised.
In principle, the high degree of security which is offered by fiber optic sealing devices depends upon several features:
A unique fingerprint is generated by the completely random pattern of the ends of the optical fibers. The uniqueness of this fingerprint is further enhanced by the imperfections in shape and optical characteristics of the individual fibers in the bundle.
It is necessary to destroy the fingerprint during any disassembly of the seal.
A formidable problem is encountered in any effort to duplicate this unique fingerprint should the original seal be replaced with a substitute.
An equally formidable problem exists in re-establishing not only the light transmitting capabilities of the individual fibers interrupted in the process of cutting the fiber bundle, but also the wide range of light intensities transmitted by the different fibers in the bundle.
The seals enable verification of the optical integrity of the fiber bundle as the whole as well as the integrity of each individual fiber in the bundle.